The present invention generally relates to wireless communication systems, and particularly relates to wireless communication receivers employing soft output value decoding.
The frequency of error in the data recovered from a received signal is a key parameter in assessing the performance of a wireless communication system. A wireless communication air interface standard typically defines the data encoding and signal modulation used by a transmitting terminal to generate a transmit signal, and the corresponding signal demodulation and decoding used by a compatible receiving terminal to recover the transmitted data. Many air interface standards include relatively sophisticated error correction encoding and interleaving techniques to enhance the reliability of the underlying data being transmitted. These techniques permit the receiving terminal to recover the data of interest from the received signal, even in the presence of bit errors in the encoded data at the receiving terminal.
One commonly used demodulation technique involves the generation of xe2x80x9csoftxe2x80x9d values, rather than hard values, at the receiving terminal. That is, instead of detecting binary ones and zeros, or discretely defined symbol values in the received signal, the receiving terminal generates a soft value that falls within the continuum of values between the discrete ideal values. The magnitude of a soft value generally reflects a xe2x80x9cconfidencexe2x80x9d level. For example, in a system where the received signal at a given instant of time is meant to indicate either a +1 or a xe2x88x921, a soft decision value might be 0.2, or xe2x88x920.8. In the first case, the soft value indicates that a +1 was recovered from the received signal but with very low confidence, while the latter case indicates the recovery of a xe2x88x921 with a relatively high confidence level. The additional information imparted by preserving the confidence level of a recovered bit or symbol may be used to enhance the performance of decoding operations involving the recovered value.
Many wireless receivers also employ some form of channel equalization to compensate the received signal for distortions caused by the communication channel. A channel equalizer essentially functions as a filter with a response that is the inverse of the channel. Commonly, the transmit signal includes a known sequence, often referred to as a xe2x80x9ctraining sequence,xe2x80x9d that the receiving terminal uses to estimate the channel response. Because channel characteristics are subject to rapid change, the transmitter provides one or more training sequences repeated at short intervals, allowing the receiving terminal to continually update its channel estimate. However, even with a frequently updated channel estimate, the accuracy of the channel estimate may be expected to degrade as it ages. Thus, data recovered from the received signal may be expected to exhibit an increasing bit error rate with increasing channel estimate age.
The present invention provides a method and apparatus for biasing soft values generated from a received signal based on an expected bit error distribution to reduce the bit error rate at the output of a channel decoder. For example, a wireless receiving terminal may demodulate a received signal to generate a sequence of soft values that are then scaled by a corresponding sequence of scaling factors before or during soft value decoding. The set of scaling factors embodies the expected bit error rate distribution for the sequence of soft values, with individual scaling factors generally having magnitudes that decrease as the expected bit error rate for the corresponding sequence position increases. That is, each bit position in the demodulator output has a corresponding scaling factor determined by the relative incidence of bit error at that bit position. Bit positions with a relatively low incidence of error will high a correspondingly high scaling factor, and bit positions with a relatively high incidence of error will have a correspondingly low scaling factor.
Soft output value biasing in accordance with the present invention is applicable to a variety of equalizers and demodulators configured to generate soft output values from a received signal. Such devices may be found in a variety of radio communication devices, such as mobile terminals for use in CDMA or TDMA communication systems. In a mobile terminal, the expected bit error rate distribution of the received signal may be preprogrammed into the terminal for use by its receiver in biasing soft values during reception operations. Alternatively, the terminal may be configured to generate the biasing information based on actual or observed bit error distributions determined from received data. The two approaches may be combined in that the terminal may be preprogrammed with default biasing information that is refined or updated based on observed bit error distribution during subsequent operation.